The present invention relates to a toothed belt transmission for driving dynamic balancing shafts in endothermic engines, of the type comprising: a driving pulley keyed on a crankshaft rotatably supported by a base of an endothermic engine; a first and a second driven pulley keyed on respective dynamic balancing shafts rotatably engaged with the base, said driven pulleys each having a pitch diameter equal to 1/2 of a pitch diameter of the driving pulley; a toothed tensioning pulley rotatably engaged with the base. A toothed transmission belt extends along a closed line extending partially over said driving, driven and tensioning pulleys, said transmission belt having a reinforcing structure formed by a plurality of cords lying consecutively side by side, incorporated in at least one layer of elastomer material in which there are defined a set of internal teeth that engages with respective toothings of the driving pulley, of the first driven pulley and of the tensioning pulley. The belt also has an external toothing that engages with a respective toothing of the second driven pulley.
Both toothings are preferably coated with at least one reinforcing layer of textile material.
As is known, many endothermic engines used in motor cars or motor cycles are provided with a pair of dynamic balancing shafts rotatably housed in the base and constituting essentially respective eccentric masses that, by rotating in respective opposite directions and at twice the speed with respect to the rate of rotation of the crankshaft, are used to eliminate vibrations due to the forces of inertia, as well as to other perturbing torques, that are well known to be generated when the engine is in operation.
To this end, the dynamic balancing shafts are driven by a belt transmission that essentially comprises a toothed belt extending, along a closed line, between respective toothed pulleys.
In particular, there is a driving pulley keyed on the crankshaft, a first and a second driven pulley respectively keyed on the dynamic balancing shafts, as well as a toothed tensioning pulley rotatably engaged with the base of the engine.
Since the dynamic balancing shafts must rotate at a rate equal to twice that of the crankshaft, the driven pulleys have a pitch diameter, and number of teeth, equal to half those of the driving pulley.
The toothed belt essentially has a reinforcing structure such as an insert formed by at least one cord for example of glass fibre helically wound in the usual manner to form a plurality of turns lying consecutively side by side along the extension of the width of the belt; such turns are incorporated in a layer of elastomer material defining, on respectively opposite parts, a toothing internal and a toothing external to the peripheral extension of the belt. Under operating conditions, the belt's internal toothing engages with the toothings of the driving pulley, of the tensioning pulley and of the first driven pulley, so that the dynamic balancing shaft associated with the latter is made to rotate in the same direction as the direction of rotation of the crankshaft. In turn, the external toothing of the belt engages with the toothing of the second driven pulley, so that the dynamic balancing shaft associated with the latter is made to rotate in the opposite direction to the direction of rotation of the crankshaft.
In the toothed belts currently used in transmissions for driving dynamic balancing shafts, the teeth constituting the individual toothings are placed consecutively side by side along a pitch of 9.525 mm, that is 3/8 of an inch.
The state of the art is standardized on this pitch size.
It has been found that current belt transmissions originate a considerable noise in operation the intensity of which is in many cases found to be unacceptable.